A block diagram of a conventional radio communications system is shown in FIG. 5.
There have been only a few kinds of communication applications used in conventional radio communications systems. Therefore, each of radio stations 10 and 20 in a conventional radio communications system has been equipped with a fixed set of hardware and software 11i and 21i (i=1, 2, - - - , N), which implements a radio communication method that satisfies communication quality required for each communication application.
Here, the communication application refers to software for exchanging information with other equipment and processing the information, and is situated at a higher layer in the radio communication method. That is, the communication application of a radio station has a function to process information followed by exchanging the information with another communication application of the same kind that is installed in another radio station through a communication path that is provided by the radio communication method. Communication applications for audio communication, communication applications for image communication, communication applications for email, communication applications for file transfer, communication applications for web communication, and the like are known as such communication applications, for example.
The radio communication method is positioned at a lower layer in the above-mentioned communication application, providing the communication path for radio communications conducted between radio stations. Specifically, a radio communication method is defined as a set of various parameters, such as a radio wave frequency to be used, a frequency bandwidth, a modulation method, a multiplexing method, a coding and decoding method, an error correction method, a radio channel structure, a data transmission format, and a communication protocol indicating a data communication procedure. And this radio communication method offers the communication path for the radio communications to be made according to such various parameters.
Namely, in the conventional radio communications system, when the communication application for voice communication and the communication application for message communication, for example are used, a radio station of the radio communications system is equipped with hardware and software for the radio communication method (radio channel structure, communication protocol and the like) for offering a communication path that satisfies communication quality demanded of the communication application for the voice communication, and hardware and software for the radio communication method (radio channel structure, communication protocol and the like) for offering a communication path that satisfies a communication quality demanded of the communication application for the message communication.
Further, since the radio station is equipped with such hardware and software fixed, they are not changed after manufacture of the radio station.
By the way, kinds of communication applications used in a radio communications system have been increasing in recent years with versatile information presented to radio communications. In such a situation, a radio station has been required to install hardware and software corresponding to many radio communication methods to be capable of handling numerous communication applications. In this case, if a radio station shall be pre-equipped with both the hardware and software corresponding to these radio communication methods like before, cost of the radio station will increase.
Furthermore, since it is rare that a user needs all kinds of the communication applications, the hardware and software corresponding to radio communication methods for unnecessary communication applications become useless.
These problems can be solved by software radio technology. The software radio technology is the technology by which hardware independent of radio communication methods and software dependent of radio communication methods realize various radio communication methods. If the radio station is equipped with the hardware independent of radio communication methods in this way, an addition of a new function and a change in a radio communication method (for example, modulation method) can be realized by downloading new software for the addition of the function and the change from a network and the like (refer to JP, 11-346186, for example).
However, with the above conventional technology, the software of a radio communication method has had to be distributed to a terminal from the network side. That is, distribution of the software of a radio communication method has been made only from a fixed station.
For this reason, unless the network side is equipped with software of a new radio communication method, communications according to the new radio communication method between the network and the terminal cannot be performed even if the terminal attempts to communicate with the network according to the radio communication method after the terminal downloads software of the new radio communication method from other nets (Internet and the like), for example.
Further, the radio station (a station on the network side) that is supposed to communicate with many radio stations (terminals) for various communication applications pursuant to user demands must anticipate beforehand and equip itself with sets of software for variety of radio communication methods suitable for various communication applications. Furthermore, it cannot communicate with a radio station at all if a communication is requested in a radio communication method suitable for a communication application, which is not prepared.